1. Field of the Invention
The invention generally relates to optically-based products and more particularly to communications over optical transmission networks.
2. Description of Related Art
Synchronous Optical Networks (SONET) have been used for inter alia, commercial telephone and data distribution services. Most of the implementations of optical data transmission for commercial use are SONET based architectures. These networks consist of an optical ring emanating from a central office (CO) terminating either at end offices or remote terminal (RT) boxes.
SONET solves the need to deploy voice and some business data service to an existing group of subscribers. However, the bandwidth for SONET was architected for traditional 64 kb/s voice services and is based on Time Domain Multiplexing (TDM). Most of the implementations were installed in the early to mid 1990s and were designed to support on the order of 10,000 subscribers per optical carrier 3 (OC3) optical ring. With business subscribers utilizing for example, T1 connections, the number of subscriber service lines is further limited.
In the context of residential broadband service, Asymmetric Digital Subscriber Line (ADSL) or other sorts of broadband access technologies operate at a bandwidth typically on the order of 100 times that of voice. Thus even though SONET rings appear to have a significant bandwidth, there is a considerable amount of oversubscription that occurs from a data perspective when trying to provide broadband telephony on a commercial scale. Gigabit Ethernet technology is a native packet transport scheme which is more efficient than a traditional TDM structure. As such it is better suited to operate in a data domain than is a TDM SONET.
Most of the existing SONET rings are built on 1310 nanometer (nm) optics. A typical OC3 ring includes a 1310 nm. laser, a photodetector that is optimized for the 1310 nm. band of light, a single mode fiber and optical splicing components. Basically, in such systems, light is transmitted from a Central Office (CO) to remote terminal #1, to remote terminal #2, to remote terminal #3, and so on, with add/drop multiplexers at each interim node.
To add another wavelength of light over the same channel (e.g., to combine a 1550 nm signal with a 1310 nm. signal in a channel), one must typically employ wave division multiplexing (WDM) hardware. The WDM hardware typically includes a Fiber Bragg Grating (FBG) or other nonlinear crystal and associated optical fibers. An erbium doped optical fiber ring can transmit both the 1310 nm. and the 1550 nm. wavelengths of light. A standard Erbium Doped Fiber Amplifier (EDFA) typically may have a bandwidth wide enough to handle either band, or anywhere in-between for that matter.
Transmission problems are seen where it is desired to use more than the 1310 nm and 1550 nm specific wavelengths. Examples include four channel add/drop multiplexers, or taking a 1310 nm., and adding four channels of 15xx wavelength hardware on top of it. For example, a four channel 1550 nm wave division multiplexor (WDM) wavelength, typically operates at 1510, 1520, 1530, and 1540 nm. Four channel 1550 operations usually have windows at each of those stages. The limitations is that if one is only adding or dropping one channel at a particular node one must have a laser that is tuned to that particular frequency, and a photodetector that looks at that particular frequency. Which means that each node has to be preset to that particular frequency. This means that the telephony installation technician has to have optics components for each wavelength at his disposal to service each different terminal. Each wavelength in which the telephony system is capable of transmitting data must have add/drop multiplexers that are tuned to that wavelength and labeled accordingly. This results in much overhead distributed among the stock room, the CO, and the other service technicians who are required to have all hardware variations with them. Additionally, the technicians must know which multiplexers to use when they try to replace terminal hardware. In other words they have to make sure the replacement for the particular multiplexer he is replacing matches the wavelength for the bad module so he does not plug in a module tuned to the wrong frequency and thus interfere with a signal of a different wavelength of some other remote terminal in the ring.
FIG. 1 is a representation of an optical ring. Central Office (CO) 100 is connected to the remote terminal equipment by fiber optic loop 500. Incumbent equipment includes a SONET OC3 ring. Due to large deployment rates in broadband, there is not enough bandwidth in the traditional SONET OC3 optical ring. Data congestion could be alleviated by a virtual point to point connection to each remote terminal without having individual fibers connect each of these sites. If this were a traditional OC3 bandwidth ring, all the bandwidth of the OC3 ring is shared over the entire ring. A parallel path could be built using the exiting fiber in the ring by using WDM. Add/drop multiplexers may be used to use put each of these Remote Terminals (RT) at different wavelengths. Assume existing SONET uses 1310 nm. Assigning RT #1 to wavelength 1510 nm., RT #2 at 1530 nm., and RT #3 at 1550 nm. These wavelengths of light are overlaid on an existing SONET ring, but use different wavelengths of light. Using traditional techniques each site would have a fixed-wavelength add/drop multiplexer and laser tuned at that particular wavelength. This would require three specific models of gear as well as deployment of all this gear at the various sites. This invention is one add/drop multiplexing module that is capable of operating at all of the wavelengths transmitted over an optical fiber and yet tunable to any specific wavelength desired at the time of deployment or replacement.
A lot of progress has been made on tunable laser technology. For example inter alia, how to tune a laser to get exactly the frequency desired. Add/drop technology in the transmit, direction is unnecessary, all that is needed is an optical combiner.
Photodiodes are generally not tunable to a particular frequency, nor do they have enough quality or selectivity to be able to isolate its own tunable band. A photo diode that is tuned to the 1300 band is likely going to respond to a signal in the 1500 band as well. A filter in front of the device is required.
Wavelength Add/drop modules are such that they will only split off a particular wavelength of light while passing remaining wavelengths. Typically the majority of the selected wavelengths energy will be split off and 99% of the other frequencies of light will pass through, making them advantageous where multiple nodes must share a single fiber run.
The difficulty with optical filters is how to make an optical filter that can be set to one of a variety of wavelengths in the field. Something that is scaleable to deployment in the field. Small enough to fit in an optical interface, and cheap enough to deploy on a mass basis. That is where that tunable wavelength filter is beneficial.
An improved add/drop multiplexer that is affordable and small enough to be deployed commercially is needed.
In one embodiment, a scheme for tuning an optical add/drop multiplexer may be used to maintain focus on the wavelength of interest, and avoid drift in signal capture due to misuse or environmental conditions and pass undesired wavelengths through to other nodes. In one example, an apparatus configured for use in the present scheme includes one add/drop multiplexing module that is tunable to the frequencies desired at the time of deployment or replacement. In this manner an existing SONET ring may employ course wave division multiplexing (two channel, potentially four channel, optical add/drop multiplexing) to add Gigabit Ethernet services on top of traditional SONET services (e.g., voice). Such a scheme may allow for an increase in useful bandwidth of approximately 10 times the bandwidth of traditional OC3.
Additional features, embodiments, and benefits of this scheme will be evident in view of the figures and detailed description presented herein.